Hillary Clinton today ended her Democratic presidential primary campaign on a winning note as she defeated her party rival Senator Bernie Sanders in the US capital, setting the stage for a fierce showdown with Republican opponent Donald Trump in November.

The result heralds the start of an epic race to the White House between Clinton, 68, and brash real estate tycoon Trump who celebrated his 70th birthday today.

"We just won Washington, DC ! Grateful to everyone who voted," Clinton tweeted.

During the primary process which started from Iowa in February and concluded in District of Columbia, Clinton won 2,219 delegates as against 1,832 by Sanders.

Clinton also received support from 581 super delegates - who are primarily the party's office bearers and elected officials - as against just 49 for Sanders.

In total Clinton now has 2800 delegates out of 4763 delegates. The half way mark is 2382. Sanders has 1832 delegates in his kitty.

Ending the primary elections on a winning note puts the former first lady on a sound footing ahead of the general election in November and the Democratic Party's convention in Philadelphia in July.

"We congratulate both Hillary Clinton and Bernie Sanders for finishing strong today in the District of Columbia after energising voters across the country with smart, substantive primary campaigns worthy of the American people," said Democratic National Committee Chair Debbie Wasserman Schultz.

"Now that our 2016 primaries are officially at their end, Democrats are ready to unify and take on both Trump and the Republican Party that he represents. At our convention in July, we're going to nominate a qualified, capable candidate who will build on the hard-won progress of the last seven years," she said.

Nearly 28 million people cast their ballots for Clinton and Sanders in Democratic primaries - 2.7 million more than the top three Republican candidates combined.

"The reason we're driving more people to the polls is simple: our candidates have shown throughout this campaign that they're committed to fighting for the hopes, dreams and aspirations of hardworking families across the country," she said.

Sanders, who is yet to drop out of the race despite a bleak chance of nomination, is to meet Hillary today. He had also met President Obama recently.